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Mechanical ventilatory assistance is now widely accepted as an effective form of therapy for respiratory failure in the neonate. Mechanical ventilators are a conspicuous and fundamental part of tertiary neonatal care. When on assisted ventilation, the newborn infant becomes part of a complex interactive system which is expected to provide adequate ventilation and gas exchange.
The overall performance of the assisted ventilatory system is determined by both physiological and mechanical factors. The physiological determinants, over which the physician has relatively little control, change with time and are difficult to define. These include the nature of the pulmonary disease, the ventilatory efforts of the infant, and many other anatomical and physiological variables. Mechanical input to the system, on the other hand, is to a large extent controlled and can be reasonably well characterized by examining the parameters of the ventilator pressure pulse. Optimal ventilatory assistance requires a balance between physiological and mechanical ventilation. This balance should insure that the infant is neither overstressed nor oversupported. Insufficient ventilatory support would place unnecessary demands on the infant's compromised respiratory system. Excessive ventilation places the infant at risk for pulmonary barotrauma and other complications of mechanical ventilation.
Intelligent management of ventilatory assistance in the neonate requires that information about the performance of the overall system be available to the clinician. Instrumentation for continuous monitoring of infants on assisted ventilation, as well as certain component variables of ventilation are known, "Instrumentation for the Continuous Measurement of Gas Exchange and Ventilation of Infants During Assisted Ventilation", K. Schulze, M. Stefanski, J. Masterson, et al., Critical Care Medicine, Vol 11, No. 11, pp. 892-896 (1983). However, at the present time, physicians rely largely on intermittent measurement of arterial blood gases to monitor the overall effects of the system on gas exchange. These measurements, while important in clinical care, have several limitations. Data acquired by such measurements provides little information about the separate contributions of the infant and the mechanical ventilator to overall ventilation and gas exchange of the infant.
Absent this information, the effects of changes in ventilator support are not as readily observable. For example, it is frequently desirable to monitor how an infant responds to respiratory therapy such as positive end expiratory pressure ("PEEP") therapy. To administer this therapy, the ventilator increases resistance to expiratory gases, thus decreasing the burden on an infants lungs.
In addition, arterial blood gas measurements are available only intermittently, which makes both trends and abrupt changes in clinical condition of the patient difficult to recognize. Continuous values are appreciably more helpful in describing the time course of changes in the patient's clinical condition.
When acquiring measurements of infant ventilation for research purposes, it customary to place the infant in a container known as a plethysmograph. With the exception of openings used for respiratory support of the infant and quantitative measurement of the infant's respiration, the interior of the plethysmograph must be isolated from the external environment. Also, for these quantitative measurements to be useful in patient care, it is desirable to configure the plethysmograph such that the sensors are in a relatively stable environment and the infant monitored remains warm and undisturbed. At the same time, however, it is essential that the infant be accessable in a very short period in the event that some emergency arises.